Hack: Any software in which a significant portion of the code was originally another program.

Hacker: Those intent upon entering an environment to which they are not entitled entry for whatever purpose [entertainment, profit, theft, prank, etc.]. Usually iterative techniques escalating to more advanced methodologies and use of devices to intercept the communications property of another.

Handshake: Parameter exchange preceding the actual data transmission.

Hash Function: (Also: Hash Algorithm) Function for computing from any input data a cryptographic checksum of given length. For good hash functions it is extremely improbable to retrieve the input data from the hash value or to find different inputs resulting in the same hash value.

Hashing: Hashing means producing hash values, also called digests, for accessing data or for security. A hash value (or digest) is a number generated from a string of text. The hash value is substantially smaller than the text itself, and is generated by a formula in such a way that it is extremely unlikely that some other text will result in the same hash value. Hashing can be used to check the integrity of data: if someone produces a hash value and sends it along with the message, the receiver can produce a hash value on receipt of the message. If the receiver's hash matches the one that was sent along with the message, the original message has not been changed.

Hash Value: (Also: Fingerprint) Result of hash function.

Hybrid Process: Combination of symmetric and asymmetric cryptography. In a first step, the message is encrypted symmetrically, the encryption key used is then encrypted asymmetrically with the recipient's public key, and sent along with the encrypted message. The recipient decrypts with his private key the symmetric encryption key and with this decrypts the message. Recommended for large amounts of data and for encryption for many recipients.

Host: Any computer that has full two-way access to other computers on the Internet. A host has a specific "local or host number" that, together with the network number, forms its unique Internet Protocol address. If you use PPP to get access to your access provider, you have a unique IP address for the duration of any connection you make to the Internet and your computer is a host for that period. In this context, a "host" is a node in a network.

Host-based Security: The technique of securing an individual system from attack. Host-based security is operating system and version dependent.

Hot Standby: A backup system configured in such a way that it may be used if the system goes down.

Hybrid Gateways: An unusual configuration with routers that maintain the complete state of the TCP/IP connections or examine the traffic to try to detect and prevent attack [may involve baston host]. If very complicated it is difficult to attach; and, difficult to maintain and audit.